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Fakulteit Ingenieurswese Faculty of Engineering Rock bed thermal storage for CSP: Design considerations K.G. Allen, T.W. von Backström, D.G. Kröger
Fakulteit Ingenieurswese
Faculty of Engineering
Rock bed thermal storage for CSP:
Design considerations
K.G. Allen, T.W. von Backström, D.G. Kröger
Presentation outline
• Context and motivation for rock beds
• Design considerations
1. Rock and containment: ‘ratcheting’
2. Air: high volumetric flow
3. Rock bed pressure drop prediction
4. Thermal characteristics, sizing and cost estimate
• Conclusion
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Need for thermal energy storage
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Two-tank molten salt
(Medrano et al., 2010. Renew. & Sust. Energy Rev. 14:56-72)
Current “state of the art”:
CSP & rock bed storage
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SUNSPOT combined cycle (Kröger)
Material suitability: thermal cycling and decomposition
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1. Thermal cycling and ‘ratcheting’
• Stress-induced failure of containment/particles
• Expansion and contraction of particles and container
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Initial state Heating Cooling
Containment vessel failure
2. Air volumetric flow rate
• Heat transfer capacity of 1 MW with ΔT = 300 °C:
• Q = ρVcΔT; air c = 1040 J/kgK, ρ = 0.67 kg/m3; molten salt c = 1200 J/kgK, ρ = 1700 kg/m3
• V air: 4.8 m3/s; V salt: 1.6 × 10-3 m3/s – a factor of 3000 …
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Bed containment: the concept of Kröger (2013)
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Free to expand/contract
Bed containment: section on A-A
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Large plenum
3. Packed bed pressure drop
Influence of:
• Particle shape
• Alignment
• Packing arrangement
• Roughness
Goal:
Prediction of rock bed pressure drop
(pumping power and cost)
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Variation of apparent friction factor
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From Allen et al., 2013. Powder Tech. 246:590-600
Field use: the volume-equivalent sphere diameter
• The rock average volume-equivalent diameter
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Counter-current packing
Crushed rock friction factor correlations
Using the volume-equivalent sphere diameter:
• Co/counter-current packing
• Cross-current packing
Where 50 < Repv < 500 and
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4 Thermal characteristics, sizing and cost estimate
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Ideal Actual
Temperature profile?
Heat transfer?
Thermocline
T ≈ 500 °C
T ≈ 20 °C
E-NTU Temperature prediction (< 75 °C)
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ΣVp/ΣAp = 1.3 mm; Bi ≈ 0.15; Re ≈ 410; Counter-current
crushed greywacke
High temperature test facility (500 - 600 °C)
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Sample measurements and E-NTU predictions
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• Importance of taking into account varying cp
• Friction factor alteration with thermal cycling
Inlet
Outlet
Size and cost estimates for D = 0.02 m, L = 7 m
Steam cycle,
MWe
Required Acs,
m2
Bed
volume, m3
Rock
volume, m3
Rock mass,
103 kg
1 (3.03 MWth) 76 (8.7x8.7) 532 319 845
10 760 (28x28) 5320 3190 8450
100 7600 (87x87) 53 200 31 900 84 535
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Steam cycle,
MWe
Rock cost,
106 R
Bed cost
(10x), 106 R
Bed cost
(3x), 106 R
Molten salt,
106 R
1 (3.03 MWth) 0.17 1.7 0.51 8.0
10 1.7 17 5.1 80
100 17 170 52 800
Storage system cost estimate
Bed sizes for different steam cycle outputs (12 hr)
Conclusions
• Ratcheting and air volumetric flow through storage
Problems can be minimised by design
• Pressure drop prediction
Importance of particle shape, roughness, arrangement
No general correlation
Correlation for specific material and packing arrangement
For irregular asymmetric particles – packing & air flow
direction crucial
• Thermal characteristics, sizing and cost estimate
Lower cost than molten salt (est. factor of 4 - 5 for scale)
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Thank you!
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Acknowledgements:
Contact details:Author A.
Thermal Energy Research Group
(STERG)
University of Stellenbosch
Stellenbosch
South Africa
+27 (0)21 808 4016
Visit us:
concentrating.sun.ac.za
blogs.sun.ac.za/STERG
Acknowledgements: Contact details:
Kenny Allen
Solar Thermal Energy Research
Group (STERG)
University of Stellenbosch
South Africa
STERG, CRSES, DST, NRF
Department of Mechanical and
Mechatronic Engineering
